The known standards for video compression, such as MPEG (Motion Picture Expert Group), used two groups 100 and 110 of steps described in FIG. 1 to compress the images of a video stream into non intra images, which is to say images which are encoded with reference to other images of the video stream:
The motion estimation 100 which consists in encoding each image from elements present in other images of the video stream (often the preceding image, the last intra image or the next intra image). The steps carried out are as follows:
step 101: cutting out the image and encoding it in blocks of a fixed size L.times.H pixels (often 16.times.16);
step 102: for each block of the image, searching in a preceding image (for example) of the floating zone of L.times.H pixels that is most similar to the block;
step 103: storage of the motion vector which indicates the motion (in spatial coordinates) between the block and the most similar zone located;
step 105: computation of the residue, which is to say the difference between the block and the most similar zone found.
motion compensation 110 which consists in compressing the residue. The steps carried out are as follows:
step 111: compression of the residue, most often thanks to a mathematical function, such as DCT (discrete cosine transform);
step 112: storage of the compressed residue and
step 113: return to step 102 to encode the following block.
The decompression of the video then takes place in the following manner:
step 121: decompression of one or several intra images (images encoded without reference to the other images of the video stream);
group of steps 130: reconstitution of the non intra images by carrying out, for each block:
step 131: localization, thanks to the motion vector, of the most similar zone of another image of the video stream;
step 132: decompression of the residue, and
step 133: adding the residue to the most similar zone to obtain the final block.
The conventional algorithms for encoding the residue (step 111) achieve suitable compression ratios only under the condition of encoding the values of the pixel in the format “Luminance Chrominance” (YUV). For a decompression on a hardware whose pixel format is “Red Green Blue” (RGB), step 133 must thus comprise moreover a color space conversion of YUV to RGB.
These known standards of video compression are very poorly adapted to the display of videos on less powerful platforms, such as portable game consoles or mobile phones.
The present invention seeks to overcome this drawback. To this end, the present invention provides, according to a first aspect, a process for the compression of a block of a size L.times.H of a sequence of images, characterized in that it comprises, in a repeating manner, for said block:
a search step, in one of the images of the sequence of images, of the zone of L.times.H pixels most similar to said block;
a step of determination whether the resemblance between said zone and said block responds to predetermined criteria;
if said resemblance responds to said criteria, a step of storage of the motion vector which indicates the distance between the block and the most similar zone found;
if said resemblance does not respond to said criteria, and if the block does not have a predetermined minimum size, a step of cutting out said block into sub-blocks and a supplemental repetition, for each of said sub-blocks, and
if said resemblance does not respond to said criteria, and if the block has a predetermined minimum size, a step of storage of the block in the course of which the block is compressed without reference to a reference image.
It is noted that the size L.times.H is generally 16.times.16 pixels and that the predetermined minimum size is preferably 1.times.1 pixel or 2.times.2 pixels, which allows numerous repetitions. Moreover, when the resemblance does not respond to the predetermined criteria and the block has a predetermined minimum size, it is not the residue which is stored but the block itself.
Thanks to these arrangements, the present invention makes possible the compression and decompression (and hence the display) of image sequences:
of a quality equivalent to the conventional coders-decoders (“codecs”),
on hardware platforms not having a sufficient machine power for conventional decompression, or even on powerful hardware platforms, but as to which it is desired to save a part of the power for another use,
with a high compression ratio.
The process provided by the inventors allows to improve considerably the efficiency of the step of motion estimation, whereby rendering useless the step of motion compensation, which is the step that is most costly during decompression.
It will be noted that by eliminating the step of motion compensation, the present invention also eliminates the step of color space conversion, necessary for certain hardwares with the conventional algorithms. As to these hardwares, their speed of decompression is thus further improved relative to the state of the art.
According to particular characteristics:
in the course of the search step, there is conjointly determined a transformation of a zone which supplies the transformed zone that is most similar to said block,
in the course of the determination step, it is determined whether the resemblance between said transformed zone and said block responds to predetermined criteria, and
in the course of the vector storage step, if said zone after said transformation is the one that bears the most resemblance, the data representative of said transformation are stored.
Thanks to these arrangements, the compression ratio is increased.
According to particular characteristics, in the course of the storage step of the block of minimal size, the block is compressed without reference to a reference image.
Thanks to these arrangements, the compression ratio and the quality of compression are increased.
According to particular characteristics, in the course of the search step, there are searched among a plurality of images of the sequence of images, the zone of L.times.H pixels the most similar to said block and in the course of the vector storage step, the data representative of the image which comprise said zone are stored.
Thanks to these arrangements, the choice between the candidate zones is greater, which allows to increase the compression ratio.
According to particular characteristics, in the course of the search step, search is conducted only in the preceding image of the sequence of images, for the zone of L.times.H pixels that is the most similar to said block.
Thanks to these arrangements, the quantity of compression is increased because it is not necessary to store data representative of the image which comprises said zone.
According to particular characteristics, in the course of the step of cutting out, said block is cut in two sub-blocks of the same dimensions.
Thanks to these arrangements, the number of sub-blocks to be treated is reduced, which increases the speed of compression.
According to particular characteristics, in the course of each cutting out step, the block or the sub-block is cut out, on the one hand, vertically and, on the other hand, horizontally and, in the course of a selection step, there is selected the cutout which optimizes the overall resemblance of the sub-blocks generated by each of said cutting out steps, with zones of said images of the image sequence.
Thanks to these arrangements, the quality of compression is improved.
According to particular characteristics, the predetermined criteria depend on the dimensions of the block in question.
The present invention provides, according to a second aspect, a device for compression of a block of a size L.times.H of a sequence of images, characterized in that it comprises a processing means adapted to control in a repeating manner for said block:
a search means which searches in one of the images in the sequence of images, for the zone L.times.H pixels that is most similar to said block,
a determination means which determines whether the resemblance between said zone and said block responds to predetermined criteria,
if said resemblance responds to said criteria, a storage means which stores the motion vector which indicates the distance between the block and the zone that has been found to be the most similar;
if said resemblance does not respond to said criteria, and if the block has no minimum predetermined size, a cutout means which cuts out said block into sub-blocks, a supplemental repetition being carried out for each of said sub-blocks and
if said resemblance does not respond to said criteria, and if the block has a predetermined minimal size, a storage means which compresses the block without reference to a reference image and stores the compressed block.
The present invention seeks, according to a third aspect, to provide a process for decompression of an image block, characterized in that it comprises, in a repeating manner:
a step of reading information representative of the block;
a step of determination whether said block is encoded in the form of several sub-blocks;
if said block is not encoded, under the form of several sub-blocks, a step of determination whether the block has a predetermined minimum size;
if said block is not encoded in the form of several sub-blocks and does not have a predetermined minimal size, a step of decompression of said block by reading of a motion vector and simple copying of the block of the same dimensions corresponding to said motion vector, in a reference image,
if said block is not encoded in the form of several sub-blocks and has a predetermined minimum size, a step of decompression of said block according to a decompression method which does not make reference to a reference image, and
if said block is encoded in the form of several sub-blocks, a return to the steps of determination for each of said sub-blocks.
The present invention seeks, according to a fourth aspect, a device for decompression of an image block, characterized in that it comprises a processing means (410) adapted to control in a repeating manner:
a reading means which reads information representative of said block;
a determination means which determines whether said block is encoded under the form of several sub-blocks;
if said block is not encoded in the form of several sub-blocks, a determination means which determines whether the block has a predetermined minimum size;
if said block is not encoded in the form of several sub-blocks and does not have a predetermined minimum size, a decompression means which decompresses said block by reading of a motion vector and simply copies the block of the same dimensions corresponding to said motion vector, in a reference image,
if said block is not encoded in the form of several sub-blocks and has a predetermined minimum size, a decompression means which decompresses said block according to a decompression method that does not make reference to a reference image, and
if said block is encoded in the form of several sub-blocks, a new repetition for each of said sub-blocks.
The advantages, objects and characteristics of the processes and of the device contemplated by the second, third and fourth aspect of the present invention being similar to those of the decompression process as briefly pointed out above, they are not repeated here.
Other advantages, objects and characteristics of the present invention will become apparent from the description which follows, given with respect to the accompanying drawings, in which: